The invention relates to a method of manufacturing bags from an initial tubular or half-tubular web of snythetic sheet material while applying the principle of the formation of loops. In a path of treatment the web is clamped at regular distances between a pair of beams or bars travelling in two separate circuits at a velocity being lower than the velocity of supply whereas the web is heatsealed and transversely perforated near the clamping zone.
According to this method the clamping beams are at the same time the sealing beams which causes the presence of a great number of these combined clamping- and sealing beams to be provided, said combined beams carrying the web of sheet material along the entire treatment path for obtaining the required welding and cooling of the seals. In this specification the word "seal" is used for the joint, or weld or seam obtained by the sealing beams. To exercise heat and pressure upon the superimposed layers of the thermoplastic material of the web does not suffice. The said seals only obtain their strength after a sufficient cooling. It was imperative to perform a heating of the combined clamping and sealing beams only in the first portion of the treatment path. In the last portion of said path the heating should be interrupted, in order to allow the seals to cool down.